Wheelchair restraint systems designed to hold a wheelchair securely in a van or bus used to transport wheelchairs and their occupants are well known in the art. Attention is directed particularly to U.S. Pat. No. 4,427,210 and its Canadian counterpart Pat. No. 1,160,907 both of which issued on 24 Jan. 1984 and which are assigned to the assignee of the present application. In this patent there is described a wheelchair tie down system comprising a first pair of straps releasably securable to the front of a wheelchair and to anchor points in the floor of the carrying vehicle; a second pair of straps releasably securable to the rear of a wheelchair and to anchor points in the floor of the carrying vehicle; and a third pair of straps designed to encircle the passenger, the free ends of which either engage the second pair of straps directly or through a bracket member, in such a way that the second and third pairs of straps lie in a straight line so that in a crash situation the force applied to the third straps by the passenger is transmitted directly to the floor and not to the relatively weak frame members of the chair. It will be appreciated that four floor anchor points must be provided in the floor of the vehicle for each wheelchair to be carried. These anchor points may be in the form of ring bolts to which dog leash type hooks at the end of the straps can be snap fastened, or they may be reinforced pockets into which a simple open hook is inserted. The former has the advantage that the lock is positive, i.e. it is unlikely to release accidentally, but it has the disadvantage that an attendant must bend down to fasten and unfasten the hook. The latter has the advantage that it is easily inserted but the disadvantage that the hook can become detached accidentally by vibration. Certain jurisdictions have enacted legislation requiring a positive locking device to secure wheelchair tie down systems. There is, therefore, a considerable need for a simple, inexpensive, floor pocket which can provide a positive lock for a conventional hook on a wheelchair tie down system.